Method for controlling the coating thickness of a coated metal strip

ABSTRACT

Method for continuously coating an advancing metal strip substrate with molten treating material and controlling the coating thickness by a fluid jet stream. The stream is projected through a discharge nozzle opening which is greater than the strip width but the effective width of the jet stream is controlled to generally correspond to the strip width by means of tracking baffles. The stream is also projected a substantially uniform distance across the effective width, as opposed to projecting the outside edges or zones of the stream a shorter distance than the middle stream zone.

United States Patent 1 La Camera et al.

[ METHOD FOR CONTROLLING THE COATING THICKNESS OF A COATED METAL STRIP[75] lnventors: Alfred F. La Camera, Youngstown; Joseph R. Semer,Columbiana, both of Ohio [73] Assignee: Youngstown Sheet and TubeCompany, Youngstown, Ohio [22] Filed: May 22, 1972 [21] Appl. No.:255,403

Related US. Application Data [62] Division of Ser. No. 110,534, Jan. 28.1971, Pat. No.

[52] US. Cl. 117/102 M, 117/102 L, 117/131, l18/8,118/63 [51] Int. ClB44d 1/06 [58] Field of Search 15/307; 117/64 R, 102 R, 117/102 M, 102L, 131; 118/8, 63

[56] References Cited UNITED STATES PATENTS 3,032,006 Kahn et al. 118/8I EFFECTIVE NOZZLE [111 3,802,911 [451 Apr.9, 1974 Fowells et al.117/102 L Senwedler et a1. 118/8 Primary Examiner-Edward G. WhitbyAttorney, Agent, or Firm-John Stelmah 5 7 ABSTRACT Method forcontinuously coating an advancing metal strip substrate with moltentreating material and controlling the coating thickness by a fluid jetstream. The stream is projected through a discharge nozzle opening whichis greater than the strip width but the effective width of the jetstream is controlled to generally correspond to the strip width by meansof tracking baffles. The stream is also projected a substantiallyuniform distance across the effective width, as opposed to projectingthe outside edges or zones of the stream a shorter distance than themiddle stream zone.

5 Claims, 3 Drawing Figures DIS CH/l RGE WIDTH PATENTEDAPR 9 1914 $802.91 1 EFFECT/VE NOZZLE DISCHARGE W/DTH EFFECTIVE NOZZLE DISCHARGE W/DTHMETHOD FOR CONTROLLING THE COATING THICKNESS OF A COATED METAL STRIPCROSS-REFERENCE TO RELATED APPLICATION This is a division of applicationSer. No. I 10,534 filed Jan. 28, 1971, now US. Pat. No. 3,687,103.

BACKGROUND OF THE INVENTION This invention relates generally to methodsfor controlling continuous coating of metal strip with molten treatingmaterial, and more particularly to methods for continuous galvanizecoating of steel strip, and most particularly to novel and improvedmethod for controlling the coating thickness across the width of thestrip.

Edge build-up on galvanized strip is a problem which has plagued thegalvanizing industry. Such build-up is considered to be excessive whenthe zinc deposit at the edges creates a spooling effect when the stripis coiled.

Others in the art have recognized this edge build-up problem and havesought to solve the problem in various ways. Considerable previous artis directed to various techniques of increasing the impact pressure atthe strip edges. Some attempt to solve the problem by providingspecially contoured discharge openings in the jet dies through whichpressurized fluid is directed toward the strip. This is undesirable ongalvanizing lines where strips of different widths are processed. Insuch cases a separate jet die set is required for each width size with aconcomitant loss in production because of additional jet die set-uptime. Others, as exemplified by the disclosure in U.S. Pat. No.2,894,856, provide means for directing separate fluid streams at thestrip edges and against the molten metal pool surface.

The art has recognized that, with the use of air knife" fluid streamsand in the absence of compensating devices, there occurs build-up of thecoating at the strip edges. It is believed that in the narrow zonesadjacent the edges of the strip, the vertical components of the fluidstreams are less than in the center portions of the strip, and that thisreduction in vertical component permits the heavier build-up at thestrip edges. In US. Pat. No. 3,526,204, it is disclosed that the wipingaction of the fluid stream be increased in the narrow zones adjacent theedges of the strip by means of a nozzle extension which brings the jetstream closer to the strip and increases the impact pressure.

In the present invention, more simple and facile methods are providedand which present a different mode of operation. Rather than increasethe wiping action at the edges, the method of this invention provides afluid stream which has a substantially uniform distribution and impactacross the width of the strip, including the narrow zones adjacent theedges.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this inventionto provide method for improving the distribution of coating materialacross the width of a metal substrate.

A specific object of this invention is to provide more simple and facilemethods for controlling a fluid stream directed toward an advancingcoated strip to deter excessive build-up at the strip edges.

A more specific object of this invention is to provide a generallyuniform fluid stream impact across the width of a coated metal strip, asdistinguished from increased impact at the strip edges, for controllingthe thickness of the coating material.

Briefly, the objects are attained by and this invention comtemplatesbaffling the discharge opening of a fluid stream nozzle for defining aneffective width of the fluid stream which corresponds generally to thewidth of the strip, and without the necessity of bringing the edges ofthe fluid stream closer, than other portions of the stream, to thestrip. Tracking is also provided so that the baffles may follow thestrip when the strip moves transversely across the width of the nozzleand to maintain a relatively constant relation between the strip widthand the effective fluid stream width.

DESCRIPTION OF THE DRAWING The invention will be more fully understoodand further objects and advantages thereof will become apparent whenreference is made to the following detailed description and to theaccompanying drawing, in which:

FIG. 1 is front elevational view of a fragmentary portion of acontinuous galvanizing line showing a steel strip exiting from themolten zinc bath together with a schematic representation of a jetnozzle forming part of the control apparatus of this invention;

FIG. 2 is side elevational view taken along line 2-2 of FIG. 1 togetherwith jet nozzles positioned on both sides of the strip; and

FIG. 3 is cross-sectional plan view, taken along line 33 of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT This invention is particularlyadapted for use in conjunction with a continuour galvanizing line andwill be described in that connection. However, it will be understoodthat the principles can be applied to other metal coating operations.

Referring to the drawing, a molten bath of coating material 10, such aszinc, is contained in tank 12. The numeral 14 designates a strip ofmaterial, such as steel, advancing out of the bath and carrying on itssurface some of the coating material.

Above the bath, the coating on the strip 14 is screeded, on opposingsides, by a pair of fluid streams emanating from discharge nozzles 16and 18. As the strip 14 advances vertically, sometimes it also has atendency to move sideways, in a direction corresponding to the lateralextent of the discharge nozzles, i.e., transverse to the advancing pathor movement of the strip. This transverse movement may be as great assix or eight inches in some cases. To compensate for this transversemovement, the nozzles 16 and 18 are made wider than the widest strip tobe processed and thus insure coverage of the entire strip width by thefluid streams.

In a representative embodiment of this invention, a pair of baffles 20is provided for each of the nozzles 16 and 18. For the purpose ofdescription, the baffles are separately designated as 20 R and 20 L. Thecorresponding support structure for each baffle is likewise designatedby similar numerals but with R and L suffixes.

Tracking means is provided for each baffle 20 of a pair to enable thebaffles to follow the transverse movement of the strip 14 and maintainthe space relationship therewith hereinafter described. The trackingmeans as illustrated comprises a bracket 24, reciprocable sup- 5 springor a motor fluid or electric. The support 26 also carries bracket 24with the baffle thereon. Thus, viewing the strp face as in FIG. 1, whenthe strip 14 shifts to the left the urging means 30 L will be compressedby the strip edge X acting on follower 28 L; concomitantly, the urgingmeans 30 R will extend causing foll5 lower 28 R to follow edge Y.

In a preferred embodiment, the follower 28 and baffle are relativelypositioned so that the distance d, from the inside baffle edge 20 e(edge nearest the strip) to the strip 14, is in the range of one-eighthto three- 20 fourths inch. Stated in another way, when the bafflescomprising a pair on one side of a strip width face, are positionedgenerally equally about the strip width, the baffles define therebetweenan effective fluid stream discharge width which is greater than thestrip width generally in the range of one-fourth inch to 1V2 inches.While the reasons are not fully known, it has been found that if thedistance d is closer than one-eighth inch considerable turbulence occursat the edge zones.

On the other hand, if the distance d is greater than three-fourths inch,the baffles do not appear to be effective toward equalizing thedistribution of the fluid stream at the edge zones. The baffles havebeen found to be most effective when the distance is in the order ofone-eighth to three-eighths inch.

The discharge opening of each nozzle may be spaced from the strip, in adirection transverse to the face of the strip, a distance s up to 1%inches, however the preferred general range is one-fourth tothree-fourths inch. The baffles 20 are positioned as close to the faceof the nozzle defining the discharge opening as possible withoutinterferring with the slidable transverse movement of the baffles acrossthe nozzle face.

The following table of data has been prepared from trials that have beenconducted. The table sets forth examples where: no baffle (NB) was usedat either strip width edge (X and Y); no baffle at one edge and a baffleB at the other edge; and a baffle B at each edge. The

line speed of the strip was in the range of to 5 feet per minute.

EDGE (l STRIP GAcE WIDTH MID: mcnes WIDTH 1. The strip edges aredesignated as X and Y; l I B des ignates that No Baffle was used at therespective edge; B designates the use of a Baffle" at the respectiveedge. I

2. d with the respective numerals designates the distance, in inchfractions, between the edge of the strip and the inside edge of thecorresponding baffie.

3. Ratio of coating weight at designated zones, oneeighth inch andone-fourth inch from respective strip edges, as compared with mid-widthcoating weight.

From the table it will be observed that at the edges where no baffle wasused, these edges were always thicker than the center in each case. Thevariation was from a +0.8 to +3.8 mils. In contrast, the variation ingage comparing baffled edge to center of strip was 0.8 to +0.5 mil; thisrepresents a substantial reduction in edge build-up and in theconcomitant propensity for spooling when the strip is coiled.

It will also be noted that the coating weightratio of non-baffled edgezone compared to outer zone is substantial, i.e., at one-eighth inchfrom edge the ratio varies from 2.16 to 3.10 and at one-eighth inch theratio varies from 1.52 to 2.28. In contrast, the baffled edge zonesvaried, at one-eighth inch, from 1.08 to 2.02 and at one-fourth inchfrom 0.80 to 1.05. This evidences substantial improvement in uniformityof coating thickness on the strip surface, and particularly in comparingthe strip edge zones with the strip mid-width.

What is claimed is:

1. In a coating process wherein a fluid stream is directed through afirst nozzle at a first side of a vertically advancing coated strip toscreed excess coating therefrom, the steps of:

maintaining an effective fluid stream width corresponding to, butgreater than, the strip width by means of baffles which are spaced aparta distance corresponding to said effective fluid stream width;

tracking transverse shifting of said strip, transverse to its advancingmovement; and

automatically and correspondingly shifting said baffles in a mannerwhereby said effective fluid stream width is maintained.

2. ln a coating process as described in claim 1,

wherein:

a fluid stream is directed at said strip through a nozzle on the sideopposing said first side, and which further comprises:

maintaining an effective tluidstream width, from the comma WEIGHTRATlOtZi) ap ears ;satire;susstsmianyzaaai raise effective fluid streamwidth from said first nozzle.

tracking movement of said strip transverse to the advancing movement;and

maintaining a generally uniform spacing between the discharge opening ofsaid nozzle and the strip face being screeded, across said effectivefluid stream width.

5. The process as described in claim 4, wherein:

said effective fluid stream is generally equally spaced across and isgreater than said strip width in the range of one-fourth inch to l /2inches.

2. In a coating process as described in claim 1, wherein: a fluid streamis directed at said strip through a nozzle on the side opposing saidfirst side, and which further comprises: maintaining an effective fluidstream width, from the opposing nozzle, substantially equal to theeffective fluid stream width from said first nozzle.
 3. In a coatingprocess as described in claim 2, wherein a separate pair of baffles isprovided for each of said first side and opposing side of the strip, andwhich further comprises: simultaneously shifting each pair of baffles inresponse to tracking of a transverse shifting of said strip.
 4. In acoating process wherein a fluid stream is directed through a nozzle atan advancing coated metal strip to screed excess coating therefrom, thesteps of: maintaining an effective fluid stream width correspondinggenerally to, but greater than, the strip width; tracking movement ofsaid strip transverse to the advancing movement; and maintaining agenerally uniform spacing between the discharge opening of said nozzleand the strip face being screeded, across said effective fluid streamwidth.
 5. The process as described in claim 4, wherein: said effectivefluid stream is generally equally spaced across and is greater than saidstrip width in the range of one-fourth inch to 1 1/2 inches.